1. Field of the Invention
The present invention relates to hair-care cosmetic compositions having dandruff formation-suppressing effect. More particularly, it relates to hair-care cosmetic compositions containing iodopropynyl butylcarbamate and/or a solution of zinc pyrithione in N-acyl ethylenediamine triacetate.
2. Description of the Prior Art
In general, "dandruff" is a keratin-like mass caused from coagulation of dead cells stripped off from head skin and sebaceous secretion of head skin. Dandruff undergoes a change of its color into reddish gradually and forms a clear boundary from normal skin. Occasionally, it causes a skin disease accompanying itch and inflammation.
It has been known, though not clearly, that such dandruff may be caused by a hormone imbalance, a nutrition imbalance, nervous stresses, biochemical changes in the epidermal tissue resulting from an excessive multiplication of head epidermal cells, growths of microorganisms on the head skin and activity increase thereof, and inflammations due to air pollutions.
Particularly, Pityrosporum ovlae, which is a parasitic on dirty head skin, has been widely recognized as a main cause of dandruff formation. Multiplication and activity increase of this dandruff-causing microorganism make dandruff formation and itches more severe.
Hence, for the hair-care cosmetics for treatment and prevention of dandruffs and itch, it is essential to incorporate ingredients capable of suppressing multiplication and activity of the Pityrosporum ovlae effectively and returning abnormal metabolism of the head epidermal cells into normal condition.
Under this circumstance, the inventors have searched for materials having antibacterial activity against the dandruff-forming strain of Pityrosporum ovlae. As a result thereof, they found unexpectedly that MIC(Minimum Inhibition Concentration) of iodopropynyl butylcarbamate against Pityrosporum ovlae is very low. This compound, iodopropynyl butylcarbamate has been used as a general preservative due to sterilizing effect against bacteria and yeast. The inventors expected this compound to be available as an antibacterial agent and tried to apply it to the hair-care products, for example shampoo. As a result, they can constitute hair-care cosmetic compositions which show excellent suppressing effect on the dandruff-formation and itch.
Further, they reviewed the synergistic effect for the combinations of iodopropynyl butylcarbamate with zinc pyrithione, the latter being a typical antibacterial compound against Pityrosporum ovlae. A hair-care composition containing iodopropynyl butylcarbamate and zinc pyrithione, although it is slightly unstable due to low solubility of zinc pyrithione in water, showed synergistic effects in antibacterial activity against Pityrosporum ovlae and thereby dandruff reduction and itch-suppressing effect.
Furthermore, in order to solve the above problem associated with low solubility of zinc pyrithione in water, they studied the mechanism of dissolution of zinc pyrithione in water.
Zinc pyrithione has a spherical shape and a particle size of 0.3.about.10 .mu.m. And it is substantially insoluble in water(10.about.20 ppm), ethanol(310 ppm) and most common organic solvents such as ethylene glycol, diethyl ether and isopropanol. Accordingly, it has been found difficult to formulate suitable cosmetic compositions containing zinc pyrithione in a dissolved form. In general, it has been accepted that such a low solubility of zinc pyrithione is due to heavy metal, zinc contained in molecule. In other words, the zinc pyrithione-dissolution needs a ligand capable of coordinate bonding with zinc contained in zinc pyrithione. As an appropriate example, amines such as diethanolamine and EDTA(ethylenediaminetetraacetic acid) have been employed. These amines have an unshared electron pair capable of coordinate bonding with zinc.(see U.S. Pat. No. 3,785,985 or U.S. Pat. No. 3,940,482).
These amines have pH 9 or high showing strong alkaline. Consequently, after dissolving zinc pyrithione, acid, for example citric acid should be added into the composition so as to adjust the pH value thereof neutral. However, by this pH change, the dissolved zinc pyrithione forms insoluble precipitates and the resulting composition becomes unstable. Further, these amines emit strong malodor, for example ammonia-like smell.
Under this circumstance, the present inventors have made research to provide a solution for the above problems associated with the low solubility of zinc pyrithione, such as instability of composition caused by precipitation of zinc pyrithione, and to find new material having unshared electron pair capable of coordinate bonding with zinc contained in zinc pyrithione and maintaining neutral in itself. As a result, they found N-acyl ethylenediamine triacetate to meet the above qualification.